The release of large amounts of energy through nuclear fission reactions is now quite well known. In general, a fissionable atom such as U.sup.233, U.sup.235, Pu.sup.239, or Pu.sup.241 absorbs a neutron in its nucleus and undergoes a nuclear disintegration. This produces on the average two fission products of lower atomic weight and great kinetic energy, and several fission neutrons also of high energy.
The kinetic energy of the fission products is quickly dissipated as heat in the nuclear fuel. If after this heat generation there is at least one net neutron remaining which induces a subsequent fission, the fission reaction becomes self-sustaining and the heat generation is continuous. The heat is removed by passing a coolant through heat exchange relationship with the fuel. The reaction may be continued as long as sufficient fissionable material exists in the fuel to override the effects of the fission products and other neutron absorbers which also may be present.
In order to maintain such fission reaction at a rate sufficient to generate useful quantities of thermal energy, nuclear reactors are presently being designed, constructed, and operated in which the fissile material (nuclear fuel) is contained in fuel elements which may have various shapes, such as plates, tubes or rods. For convenience, these fuel elements will hereinafter be referred to as fuel rods. These fuel rods are usually provided on their external surfaces with a corrosion-resistant non-reactive cladding. The fuel rods are grouped together at fixed distances from each other in a coolant flow channel or region as a fuel assembly or bundle. A sufficient number of the fuel assemblies are arranged to form a nuclear reactor core capable of the self-sustained fission reaction discussed above.
A typical fuel assembly is formed, for example, by an 8 .times. 8 array of spaced fuel rods, the rods being several feet in length, approximately one-half inch in diameter and spaced from each other by a fraction of an inch. To prevent such elongated rods from touching one another, through bowing and vibration during reactor operation, it is necessary to retain the rods in spaced relation by a plurality of fuel rod spacers positioned along the length of the fuel rods.
A variety of fuel rod spacers have been proposed and used. For example, fuel rod spacers are shown in U.S. Pat. Nos. 3,350,275 and 3,654,077, both assigned to the assignee of the present invention. Although the fuel rod spacers disclosed and claimed in the aforementioned patents function as intended, the desirability of improving the spacing characteristics and the thermal-hydraulic performance of the spacer will be evident to those skilled in the art. During reactor operation, the flow channel is deformed outwardly by an internal-to-external pressure differential. The effect of this outward deformation is to permit the spacing between the corner fuel rod and the flow channel to be reduced. It is an object of the present invention to prevent this reduction in spacing from occurring. Also, the thermal or power generation capability of the fuel is limited by the capability of the coolant efficiently to conduct heat from the fuel rods. The heat conduction capability is, in turn, influenced by the geometry of the spacer. Thus, another object of the invention is a spacer which provides improved thermal performance.